1. Field of the Invention
This invention relates to a method for producing mono-crystalline alumina grains to be used for grinding materials, ceramic materials, refractory materials and so forth.
2. Prior Arts
Conventionally, mainly employed for the grinding materials, ceramic materials and refractory materials has been fused alumina. Recently, however, these materials are liable to be used under severe conditions, and consequently conventional fused alumina is hardly applicable from the view points of electrical insulating properties, grinding efficiency and corrosion resistance thereof.
In order to solve the above problems, there have been developed white fused alumina containing less alkali and white fused alumina containing less .beta.-alumina. Said alumina have been produced by various methods, for example, employing alumina materials of low alkali content; fusing bayer alumina in the presence of carbon as shown in the Japanese patent publication No. 46-5577; fusing bayer alumina in the presence of metal aluminium grains or metal aluminium powders as shown in the Japanese patent publication 48-38203; fusing bayer alumina in the presence of grains or powders of metallic silicon alloys as shown in the Japanese patent publication No. 52-12204; fusing bayer alumina in the presence of boric acid or boric acid anhydride to prepare alumina spherical hollow grains and subjecting said grains to a treatment by inorganic acids such as hydrochloric acid, and so forth.
These methods, however, were expensive in view of the materials used and the manufacturing processes. In addition, a product obtained is liable to be degraded by coloring caused by an unreacted additive remained in the product.
Japanese patent application laid open under No. 53-79797 discloses a method for producing electro-fused alumina having a purity more than 99.9% by employing the processes comprising electro-fusing alumina materials in the presence of silicon dioxide(SiO.sub.2) and sodium oxide(Na.sub.2 O), cooling and solidifying the fused material into an ingot, crushing said ingot and applying hydrofluoric acid and hydrochloric acid to treat a silica glass phase. This method, however, is deficient in handling of the hydrofluoric acid which is a very strong acid.
Japanese patent publication No. 44-16518 discloses a method for producing electro-fused white alumina(WA) of high toughness and low alkali content by electro-fusing bayer alumina in the presence of aluminium fluoride. But fused alumina of mono-crystalline grain has not been provided.
In the field of grinding materials, especially for grinding metals which are hardly ground, mono-crystalline grains HA have been willingly used because ordinal white fused alumina WA is deficient in grinding efficiency. WA, HA and other artificial abrasives are stipulated in the Japanese Industrial Standard, JIS R 6111. Mono-crystalline grains HA have such merits that there are little boundaries, cracks, open pores and closed pores in the grains compared with regular fused alumina A and white fused alumina WA.
A general method which is well known in the grinding industry for producing mono-crystalline grains is a electro-fusing method of alumina materials in the presence of iron sulfide, sulfur, sulfide mineral and scrap iron. On the other hand Japanese patent publication No. 46-9396 discloses a method comprising electro-fusing bayer alumina in the presence of sulfur or iron sulfide and further adding in molten contents titania(TiO.sub.2), chromic oxide (Cr.sub.2 O.sub.3) and ferric oxide (Fe.sub.2 O.sub.3). These methods, however, using sulfides, harmful gases such as SO.sub.2, SO.sub.3, H.sub.2 S are induced so that a special treatment for solving the problems of environmental pollution is needed. Japanese patent publication No. 59-18359 discloses a method for producing mono-crystalline alumina grains which are obtained by cooling quickly fused alumina as far as an inside thereof before solidification to create a plurality of crystal nuclei over the whole region of the fused alumina and after solidifying crushing mechanically. In this method, however, yield of mono-crystalline grains is not so high, that is 30-45%. On the other hand it is well known that regular fused alumina can be modified so as to be applicable to various grinding purposes by electro-fusing alumina materials in the presence of a little amount of chromium oxide, titanium oxide, vanadium oxide etc. which form a solid solution in the alumina crystal grains. In the conventional methods for producing mono-crystalline alumina grains, materials are fused accompanying a partial reduction with added cokes. In addition to said reducing atmosphere, as impurities gather at grain boundaries, it is very difficult to introduce such modifiers in the crystal grains in the form of solid solution. If possible, yield is very low.